An initial investigation of a 20,000 barrel pipeline spill in North Dakota points to corrosion, officials said. The owner, Tesoro Logistics, has not speculated on a cause.

Tesoro Logistics LP, headquartered in San Antonio, TX, owns the 35-mile pipeline, which carries crude oil from the Bakken shale play to the Stampede rail facility outside Columbus, ND.

Mum for 11 Days

The North Dakota State Health Department was told about the spill on Sept. 29, but said they didn't announce it publicly for 11 days because they thought it was a small spill of around 750 barrels of oil.

However, an estimated 20,600 barrels of oil over 7.3 acres of land had spilled, making it one of the largest spills in North Dakota's history, according to media reports.

The Public Service Commission is responsible for enforcing safety requirements for intrastate distribution and transmission of natural gas and establishing and enforcing rates or charges and regulations by pipeline carriers, according to its website.

The U.S. Department of Transportation's Pipeline and Hazardous Materials Safety Administration is responsible for investigating the cause of the spill and oversees the pipeline's operation.

Kalk told Reuters that he expects the federal investigation to discuss why pumping stations failed to indicate a drop in pressure and why maintenance records do not show problems on the line.

'Unsubstantiated' Reports

A press release from Tesoro said the pipeline released approximately 20,000 barrels of crude oil in a rural field northeast of Tioga, ND, but the company did not announce a possible cause.

"Tesoro will not speculate on the cause of the leak as it is currently under investigation," Tina Barbee, a media relations representative for the company told PaintSquare News in an email on Monday (Oct. 14).

"Any reported causes that have been made by any person or group at this point are unsubstantiated and purely speculative," Barbee said, adding that the company is focused on cleanup, repair and remediation and did not have any additional updates.

A farmer reported the oil spill on Sept. 29, but the state's Department of Health did not announce it publicly until Oct. 10, saying that it initially thought it was a smaller spill.

According to Tesoro, the affected segment of the pipeline has been shut down, the release has been stopped and repairs are currently underway. The company says that no injuries or impacts to water, wildlife or the surrounding environment have been reported.

Tesoro says it expects that remediation of the incident will cost $4 million and an internal investigation is underway to determine the cause.

"Protection and care of the environment are fundamental to our core values, and we deeply regret any impact to the landowner," said Greg Goff, Tesoro's chairman and CEO. "We will continue to work tirelessly to fully remediate the release area."

Requests for more information from PHMSA, the Public Service Commission and the state's Department of Health were not immediately returned on Monday.

About the Pipeline

The 35-mile pipeline runs from Tioga to Black Slough in North Dakota and was built by BP Plc in 1993. Tesoro bought the pipeline and refinery from BP in 2001. It is part of Tesoro's "High Plains" pipeline system in North Dakota and Montana.